This invention relates generally to drilling of wells and production from wells.
Generally, wells are drilled in a slightly over-balanced condition where the weight of the drilling fluid used is only slightly over the pore pressure of the rocks being drilled.
Drilling mud is pumped down the drill string to a drill bit and used to lubricate and cool the drill bit and remove drilled cuttings from the hole while it is being drilled. The viscous drilling mud carries the drilled cuttings upwardly on the outside and around the drill string.
In a balanced situation, the density of the mud going downwardly to the drill bit and the mud passing upwardly from the drill bit is substantially the same. This has the benefit of reducing the likelihood of a so-called kick. In a kick situation, the downward pressure of the drilling mud column is not sufficient to balance the pore pressure in the rocks being drilled, for example of gas or other fluid, which is encountered in a formation. As a result, the well may blowout (if an effective blow out preventer (BOP) is not fitted to the well) which is an extremely dangerous condition.
In underbalanced drilling, the aim is to deliberately create the situation described above. Namely, the density or equivalent circulating density of the upwardly returning mud is below the pore pressure of the rock being drilled, causing gas, oil, or water in the rock to enter the well-bore from the rock being drilled. This may also result in increased drilling rates but also the well to flow if the rock permeability and porosity allowed sufficient fluids to enter the well-bore.
In this drilling environment it is general practice to provide a variety of blowout preventers to control any loss of control incidents or blowouts that may occur.
A variety of techniques have been utilized for underbalanced or dual gradient drilling. Generally, they involve providing a density lowering component to the returning drilling mud. Gases, seawater, and glass beads have been injected into the returning mud flow to reduce its density.
In deep subsea applications, a number of problems may arise. Because of the pressures involved, everything becomes significantly more complicated. The pressure that bears down on the formation includes the weight of the drilling mud, whereas the pressure in the shallow formations is dictated by the weight of seawater above the formation. Because of the higher pressures involved, the drilling mud may actually be injected into the formation, fracture it and may even clog or otherwise foul the formation itself, severely impairing potential hydrocarbon production.